It is known how to provide power plants with a boiler for steam generation and a flue gas treatment system downstream of the boiler. The flue gas treatment systems are used to remove components that can be dangerous for the environment from the flue gas that is discharged into the atmosphere, such as carbon dioxide.
A number of different systems are known to remove carbon dioxide from flue gas, such as the carbonator/calciner loop. According to this system, flue gas is supplied into the carbonator such that the CO2 reacts with a sorbent (for example CaO) to generate CaCO3; this reaction is usually carried out at a temperature between 550-650° C.
The CaCO3 is supplied to the calciner, where at a temperature usually between 850-1050° C. it is decomposed into CO2 that is stored or used in other way and CaO that is recycled back into the carbonator.
EP 2 559 475 discloses a system having the features described above.
In addition to CO2, usually other gaseous components contained in the flue gas must be removed before discharging the flue gas into the atmosphere.
For example, often the fuel contains sulphur that during combustion generate sulphur components that shall be removed from the flue gas before discharging the flue gas into the atmosphere.
US2002/0037246 discloses how to supply CaO in a combustion chamber (for example of a circulating fluidized bed) in order to remove SO2 and, in addition, this document also discloses how to use the surplus CaO in the ash to capture CO2 from the flue gas. US2002/0037246 does not deal with the thermodynamic optimization of a boiler and, in particular, of a boiler provided with a flue gas treatment system.
FIG. 1 shows an example of a boiler 1 with a flue gas treatment system 2. The flue gas treatment system 2 comprises a heat exchanger 3 immediately downstream of the boiler 1. The heat exchanger 3 is an air pre-heater for pre-heating the combustion air that is supplied into the boiler. In different embodiments the heat exchanger 3 can be downstream of the boiler or it can be within the boiler. Downstream of the heat exchanger 3 a desulphurization unit 4, another heat exchanger 5 and a carbonator/calciner loop 6 are provided. The heat exchanger 5 is for heating the flue gas up to a temperature that fits the temperature of the carbonator into which the flue gas is fed. From the carbonator/calciner loop 6 a CO2 stream 7 and a vented gas stream 8 (comprising e.g. N2, Argon) are discharged.
In addition, a dust removal unit 9 is often provided downstream of the boiler 1.
Thermodynamic optimization is achieved by optimizing the boiler; thus the boiler 1 and the heat exchanger 3 (air pre-heater) are optimized in order to increase thermal efficiency.
Nevertheless, even if the thermal efficiency of the boiler is optimized, the global thermal efficiency of the boiler and flue gas treatment system could not be optimized.
In fact after the flue gas is cooled at the heat exchanger 3 (in order to heat the combustion air supplied into the boiler), the flue gas must be sensibly heated again at the heat exchanger 5. This large heating can affect the global thermal efficiency of the boiler and flue gas treatment system.